Mass spectrometers are workhorse instruments finding applications in many commercial and military markets, with potential for use in domestic markets as well. A mass spectrometer is able to sample, in situ, the atmosphere in which it is placed and provide a reading of the atomic and molecular species (and any positive or negative ions) present in that atmosphere and of the absolute abundance of these species.
There are many types of mass spectrometers, such as magnetic sector, Paul or Penning ion trap, trochoidal monochromator, and the like. One popular type of mass spectrometer is the quadrupole mass spectrometer (QMS), first proposed by W. Paul (1958). In general, the QMS separates ions with different masses by applying a direct current voltage and a radio frequency ("rf") voltage on four rods having hyperbolic or circular cross sections and an axis equidistant from each rod. Opposite rods have identical potentials. The electric potential in the quadrupole is a quadratic function of the coordinates.
Ions are introduced in a longitudinal direction through a circular entrance aperture located at the ends of the rods and centered on the midpoints between rods. Ions are deflected by the field depending on their atomic mass-to-charge (m/z) ratio. By selecting the applied voltage amplitude and frequency of the rf signal, only ions of a selected m/z ratio exit the QMS along the axis of a quadrupole at the opposite end and are detected. Ions having other m/z ratios either impact the rods and are neutralized or deflect away from the centerline axis of the quadrupoles.
As explained in Boumsellek, et al. (1993), a solution of Mathieu's differential equations of motion in the case of round rods provides that to select ions with a m/z ratio using an rf signal of frequency f and rods separated by a contained circle of radius distance R.sub.0 the peak rf voltage V.sub.0 and dc voltage U.sub.0 should be as follows: EQU V.sub.0 =7.233mf.sup.2 R.sup.2.sub.0 EQU U.sub.0 =1.213mf.sup.2 R.sup.2.sub.0.
Conventional QMS's weigh several kilograms, have volumes of the order of 10.sup.4 cm.sup.3, and require 50-100 watts of power. Further, these devices usually operate at vacua in the range of 10.sup.-6 -10.sup.-8 torr in order that the mean free path be comparable to the instrument dimensions, and where secondary ion-molecule collisions cannot occur. Commercial QMS's of this design have been used for characterizing trace components in the atmosphere (environmental monitoring), automobile exhausts, chemical-vapor deposition, plasma processing, and explosives/controlled-substances detection (forensic applications). However, such conventional QMS's are not suitable for spacecraft life-support systems and certain national defense missions where they have the disadvantages of relatively large mass, volume, and power requirements. A small, low-power QMS would find a myriad of applications in factory air-quality monitoring, pollution detection in homes and cars, protection of military sites, and protection of public buildings and transportation systems (e.g., airports, subways, and harbors) against terrorist activities.
One type of miniature QMS (U.S. Pat. No. 5,401,962) was developed by Ferran Scientific, Inc., San Diego, Calif. and includes a miniature array of sixteen rods comprising nine individual quadrupoles. The rods are supported only at the detector end of the QMS by means of powdered glass that is heated and cooled to form a solid support structure. The electric potential and rf voltage are applied by the use of springs contacting the rods. The Ferran QMS dimensions are approximately 2 cm diameter by 5 cm long, including a gas ionizer and detector, and has an estimated mass of 50 grams. The reduced size of the Ferran QMS results in several advantages over existing QMS's, including a reduced power consumption and a higher operating pressure.
The Ferran QMS has a resolution of approximately 1.5 amu in the mass range 1-95 amu. This is a relatively low resolution for a QMS, making the miniature Ferran QMS useful for commercial processing (e.g., chemical-vapor deposition, blood plasma monitoring) but not for applications that require accurate mass separation, such as in analytical chemistry and in spacecraft life-support systems. Boumsellek et al. (1993) traced the low resolution to the fact that the rods were aligned only to within a .+-.3% accuracy, whereas an alignment accuracy in the range of .+-.0.1% is necessary for a high resolution QMS.
A separate miniature QMS (U.S. Pat. Nos. 5,596,193 and 5,719,393) was developed by the Jet Propulsion Laboratory (JPL), California Institute of Technology to address the continuing need for a reduced size QMS having an acceptable rod alignment. The JPL QMS provides improved resolution over the Ferran QMS due to improved accuracy in rod alignment. As may be appreciated, the accurate positioning and alignment of individual miniature rods in an array significantly increases the cost of manufacturing due to the increased time and specialized equipment required for precisely aligning separate miniature rods. As the size of the rods is further reduced, the complexity, difficulty and expense of rod positioning and alignment increases. In this regard, there is a need for a small QMS having high resolution that may be made by simpler and less expensive manufacturing process.